This invention relates to a transistor differential amplifier circuit which produces output pulse signals from sine wave input signals, and, more particularly, to a transistor differential amplifier circuit which provides a controlled duty radio of the output pulse signals at a desired value.
In a brushless motor using Hall-effect elements, for example, a transistor differential amplifier has generally been utilized to amplify sine wave signals generated by the Hall-effect elements. Using such a transistor differential amplifier circuit having a sufficiently high value of amplification factor, pulse shape output signals can be obtained from the sine wave input signals.
These pulse shape signals are supplied to the field coils of the brushless motor as their field currents, whereby rotating magnetic fields for rotating a rotor are produced. In this case, it becomes necessary to vary the duty ratio of the pulse shape signals obtained from the differential amplifier in order to control the rotating magnetic fields. According to the conventional differential amplifier circuit, however, it has been difficult to vary the duty ratio of the pulse shape output signals delivered therefrom. In the previous amplifier circuitry, the derived pulse shape output signals have a duty ratio fixed at 50% of the input sine wave signal since these pulse shape output signals have been delivered corresponding to the half cycle of the input sine wave signals only.